If you run to rich there will be NO deposits on the piston or anywhere else in the combustion chamber.
The ideal setting should yield a slightly brown/golden surface that can be wiped away with paper.

What does the filament in the plug look like?

If it's shiny and straight you are running on the rich side.
If it is slightly silver-dull you have tuned the engine for maximum power.
If it's pushed-in/distorted you are lean.

This assumes you put a new plug in before this run. Otherwise it will be hard to diagnose.